Network Working Group J. Rajahalme
Request for Comments: 3697 Nokia
Category: Standards Track A. Conta
Transwitch
B. Carpenter
IBM
S. Deering
Cisco
March 2004
IPv6 Flow Label Specification
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document specifies the IPv6 Flow Label field and the minimum
requirements for IPv6 source nodes labeling flows, IPv6 nodes
forwarding labeled packets, and flow state establishment methods.
Even when mentioned as examples of possible uses of the flow
labeling, more detailed requirements for specific use cases are out
of scope for this document.
The usage of the Flow Label field enables efficient IPv6 flow
classification based only on IPv6 main header fields in fixed
positions.
1. Introduction
A flow is a sequence of packets sent from a particular source to a
particular unicast, anycast, or multicast destination that the source
desires to label as a flow. A flow could consist of all packets in a
specific transport connection or a media stream. However, a flow is
not necessarily 1:1 mapped to a transport connection.
Rajahalme, et al. Standards Track [Page 1]RFC 3697 IPv6 Flow Label Specification March 2004
Traditionally, flow classifiers have been based on the 5-tuple of the
source and destination addresses, ports, and the transport protocol
type. However, some of these fields may be unavailable due to either
fragmentation or encryption, or locating them past a chain of IPv6
option headers may be inefficient. Additionally, if classifiers
depend only on IP layer headers, later introduction of alternative
transport layer protocols will be easier.
The usage of the 3-tuple of the Flow Label and the Source and
Destination Address fields enables efficient IPv6 flow
classification, where only IPv6 main header fields in fixed positions
are used.
The minimum level of IPv6 flow support consists of labeling the
flows. IPv6 source nodes supporting the flow labeling MUST be able
to label known flows (e.g., TCP connections, application streams),
even if the node itself would not require any flow-specific
treatment. Doing this enables load spreading and receiver oriented
resource reservations, for example. Node requirements for flow
labeling are given in section 3.
Specific flow state establishment methods and the related service
models are out of scope for this specification, but the generic
requirements enabling co-existence of different methods in IPv6 nodes
are set forth in section 4. The associated scaling characteristics
(such as nodes involved in state establishment, amount of state
maintained by them, and state growth function) will be specific to
particular service models.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119
[KEYWORDS].
2. IPv6 Flow Label Specification
The 20-bit Flow Label field in the IPv6 header [IPv6] is used by a
source to label packets of a flow. A Flow Label of zero is used to
indicate packets not part of any flow. Packet classifiers use the
triplet of Flow Label, Source Address, and Destination Address fields
to identify which flow a particular packet belongs to. Packets are
processed in a flow-specific manner by the nodes that have been set
up with flow-specific state. The nature of the specific treatment
and the methods for the flow state establishment are out of scope for
this specification.
The Flow Label value set by the source MUST be delivered unchanged to
the destination node(s).
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IPv6 nodes MUST NOT assume any mathematical or other properties of
the Flow Label values assigned by source nodes. Router performance